1. Field of the Invention
The invention relates generally to a crane hook assembly for lifting heavy, elongated loads and to methods for safely lifting such loads. Particularly, the present invention relates to a specialized crane hook assembly for lifting the elongated, heavy sections, which when assembled form the high support tower for a wind powered electrical generator.
2. Description of the Related Art
Wind powered generators are mounted at the top of high support towers that are commonly over two hundred feet high so as to catch the high surface winds. The towers are installed in open country, usually far distant from heavily populated areas, and all the components of the generator and tower must be transported to the site by rail or large truck and there unloaded by cranes equipped for the unloading process. A large, high capacity generator unit is typically mounted on the top of such a tower, approximately 240 feet above ground level. The typical tower is formed from three long cylindrical tower sections, each section having a length of about 82 feet. The lowest section is quite massive with a fourteen foot diameter and can weigh about eighty tons. The middle section is slightly less in weight and has some taper from bottom to top. The upper section tapers further to a final diameter of about 9 feet at its upper end. Typically, each tower section has an annular mounting flange at each end, with the flange extending radially inwardly and outwardly from the cylindrical tower section wall at a right angle to the tower section's central longitudinal axis. Each annular flange has a multiplicity of bolt apertures positioned about its outer perimeter, and these apertures align with the apertures of an adjoining tower section's flange to allow adjacent tower sections to be bolted together end-to-end when the tower is erected at the construction site.
Each of the three tower sections is transported to the tower construction site on separate semi-trucks or rail cars, and the sections are delivered on a rigorous schedule which assures that all the trucks or rail cars will be available for prompt unloading so the trucks or rail cars can return at once to normal service. Because of their great size, it is usually necessary to lift each of the tower sections from the truck bed or rail car using two separate cranes, one being positioned at each end of the tower section.
Previous devices and methods for lifting and unloading the tower sections have utilized steel or nylon web slings positioned around the outer periphery of the tower section near each end of the tower. Another known hook assembly utilizes a pair of crane hook assemblies, wherein a hook assembly is bolted to the flanges at each end of the tower section using a multiplicity of bolts and nuts so as to provide a relatively fail safe engagement. Additional known crane hook assemblies were developed with structure which engaged the inner circumference of the annular flange at each end of the tower section.
The present invention addresses limitations associated with the prior art.